feat: add broadcasts.cancel() method

[dielduarte]

Summary

• Adds BroadcastsSvc::cancel(broadcast_id), calling POST /broadcasts/:id/cancel to cancel a queued or scheduled broadcast.
• Adds CancelBroadcastResponse type (id) and re-exports it from lib.rs, mirroring SendBroadcastResponse/UpdateBroadcastResponse.
• Adds a parse_cancel_broadcast_response_test deserialization test, matching the existing parse_broadcast_test pattern.

Part of the broadcast cancel rollout: resend/resend-monorepo#8126, resend/resend-openapi#85, resend/resend-node#1059, resend/resend-docs#1722, resend/resend-go#144, resend/resend-java#122, resend/resend-php#135, resend/resend-python#250, resend/resend-ruby#216.

Test plan

• cargo check --lib --tests (default features) — clean
• cargo clippy --lib --tests --no-default-features --features blocking,native-tls -- -D warnings — clean
• cargo test --lib broadcasts:: — 2 passed, 3 pre-existing ignored (live-API integration tests)
• cargo fmt --check — clean

🤖 Generated with Claude Code

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Summary by cubic

Adds BroadcastsSvc::cancel() to cancel queued or scheduled broadcasts via POST /broadcasts/:id/cancel. Exposes CancelBroadcastResponse and re-exports it in lib.rs to match other broadcast responses.

• New Features
  • Implement cancel(broadcast_id); sends POST with no body and returns CancelBroadcastResponse.
  • Add CancelBroadcastResponse { id }; re-exported from lib.rs.
  • Add parse_cancel_broadcast_response_test for deserialization.

^{Written for commit e49fdc3. Summary will update on new commits.}

[klotty]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: Adds a new cancel() API method and response type mirroring existing broadcast methods, with a deserialization test. Additive, well-scoped change with no operational or contract tradeoffs.

Re-trigger cubic</sub>